network - MP socket free & abort interactions, so_state

network - MP socket free & abort interactions, so_state

* Add so_refs and ref-count the socket structure to deal with
  MP races on sofree().

* Ref the socket structure for all soabort() operations (they are usually
  asynchronous).  The netmsg_pru_abort() handler will sofree() the ref
  after calling the protocol stack's abort function.

* Use atomic ops to set and clear bits in so_state, because it is modified
  by both the fronttend and the backend.

* Remove numerous critical sections that are no longer effective.

* Protect the accept queues with so_rcv.ssb_token.

* Protect after-the-fact calls to soisdisconnected() with a soreference()
  to avoid use-after-free cases.

* Wrap unix domain, mroute, div, raw, and key sockets/protocols with their
  own private tokens.